In digital video communication systems, variable bit-rate (VBR) transmission can provide constant video quality and can improve bandwidth efficiency through statistical multiplexing. These benefits can be realized using packet networks. However, most video conferencing terminals in use today are designed for operation over constant bit-rate (CBR) circuit-switched networks. As a result, no statistical gains can be obtained. Furthermore, when complex motion is encountered or during a scene change or a screen refresh, the video quality may be substantially degraded.
Asynchronous Transfer Mode (ATM) networks can statistically multiplex VBR video bitstreams with other traffic types (data, voice, image) while providing a quality-of-service according to the needs of each traffic type. It is expected that in the future videoconferencing terminals will be designed for operation over ATM networks and thus will support VBR transmission. However, CBR videoconferencing terminals are expected to remain in use.
CBR video signals can be carried over ATM networks using so-called circuit emulation. However, circuit-emulation has many of the same drawbacks of circuit-switching: it does not allow statistical multiplexing and results in variable video quality. Moreover, the overhead of packet headers further reduces the transmission efficiency.
Several video compression standards have been defined in recent years by various standards organizations. These standards specify the syntax for the bitstream transmitted by the video encoder. A decoder can reconstruct the compressed video signal from the received bitstream using its knowledge of the standard syntax.
Thus, there is a need for a system that improves the video quality and bandwidth efficiency in transporting video signals that are generated by CBR terminals.